1. Field of the Invention
The present invention relates generally to the field of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.) and, in particular to a new and useful method and apparatus for; (i) reducing the levels of one or more gas phase selenium compounds and/or one or more other RCRA metals, or RCRA metal compounds (regardless of whether such other RCRA metals or RCRA metal compounds are in the gas phase or some other phase); (ii) capturing, sequestering and/or controlling one or more gas phase selenium compound and/or one or more other RCRA metals, or RCRA metal compounds (regardless of whether such other RCRA metals or RCRA metal compounds are in the gas phase or some other phase) in a flue gas stream and/or in one or more pieces of emission control technology; and/or (iii) capturing, sequestering and/or controlling one or more gas phase selenium compound and/or one or more other RCRA metals, or RCRA metal compounds (regardless of whether such other RCRA metals or RCRA metal compounds are in the gas phase or some other phase) in a flue gas stream prior to desulfurization and/or in one or more pieces of emission control technology prior to one or more desulfurization units.
2. Description of the Related Art
As is known to those of skill in the art, fossil fuel-based electric plants that utilize steam to generate electric power discharge various substances into the environment including, but not limited to, arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), selenium (Se) and/or silver (Ag). Many of these metals and/or RCRA metal compounds, once in the environment, remain there for years. Some of the current emission regulations relating to various allowable emission levels for some of these substances were last updated in 1982 and as a result do not adequately address various metal discharges.
New technologies for generating electric power and the widespread implementation of air pollution controls over the last 30 years have altered existing wastewater streams or created new wastewater streams at many power plants, particularly coal-fired plants. Given this, the 2015 EPA Finalized Rules address these changes in the power generating industry and in particular focuses on wastewater streams from various emission control technologies.
In light of various new guidelines such as the 2015 EPA Finalized Rules, one of the new Effluent Limit Guidelines (ELG) relates to the amount of Resource Conservation and Recovery Act (i.e., RCRA) metals that can be emitted in a wastewater stream. As such, it has become, or eventually will become, desirable to control the amount of one or more of arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), selenium (Se) and/or silver (Ag) (collectively referred to as the RCRA metals) that is/are emitted in one or more wastewater streams generated during various emission control technologies. Various potential emission sources of one or more RCRA metals include one or more effluent streams, or wastewater streams, generated from one or more pieces of emission control technology utilized to clean the flue gas of various fossil fuel burning power plants (e.g., a coal-fired power plant). Given this, and in light of the newly proposed guidelines, only a certain concentration of each metal is permitted in waste, it has now become desirable to develop cost effective methods for capturing one or more of the RCRA metals. While not wishing to be bound to any one ELG Guideline, it is believed that the currently proposed, or even final, long-term average ELGs for various RCRA metals from wastewater from existing wet FGD units will be, or are, arsenic —5.98 μg/L; mercury —159 ng/L; and selenium —7.5 μg/L. It should be noted that other RCRA metal ELGs would apply to other emission control wastewater streams and/or other emission situations, and thus the above ELGs are to be viewed as exemplary and non-limiting.
While not wishing to be bound to any one theory, it is believed that gas phase selenium in combustion flue gases exists mainly as SeO2. While it is known that some of the gaseous SeO2 gets adsorbed on fly ash surfaces, much of the gas phase selenium will continue to remain in the gas phase and then enter one or more wet FGD scrubber units as selenium vapor. Also entering such one or more wet scrubber units will be any selenium that is present on any small particulate matter that may have passed through any upstream particulate control device (e.g., one or more electrostatic precipitators, one or more baghouses and/or any other type of particulate control technology). During the desulfurization process, the wet FGD scrubber further cools the flue gas thereby resulting in the condensation of one or more gas phase selenium compounds into the water and/or aqueous slurry present in the wet FGD unit. This in turn results in the eventual presence of one or more selenium compounds in any wastewater stream that is generated during the desulfurization process.
Again, while not wishing to be bound to any one theory, it is believed that the dominant oxidation forms of selenium within the wet FGD slurry and/or any wastewater stream therefrom are one or more selenite (SeO32−) compounds and one or more selenate (SeO42−) compounds, also referred to as selenium oxyanions. Depending on the speciated form of such selenium compounds (in particular the generally more water soluble selenate compounds), treatment options can get complicated as dissolved selenium compounds in either the wet FGD slurry and/or wastewater stream cannot typically be removed by conventional chemical precipitation waste treatment technologies below current ELG (7.5 μg/L). Untreated selenium compounds are then present in the wastewater stream from the wet FGD thereby requiring special and expensive treatment equipment to remove same. Such processes can include, but are not limited to, various water purification processes known to those of skill in the art such as bio-reactors, one or more distillation processes (e.g., multi-stage flash distillation (MSF), multiple-effect distillation (MED/ME), vapor-compression (VC), etc.); one or more ion exchange processes, one or more membrane-based processes (e.g., electrodialysis reversal (EDR), reverse osmosis (RO), nanofiltration (NF), membrane distillation (MD), forward osmosis (FO), etc,).
Given the newly proposed effluent limit for selenium of 7.5 μg/L or ppb total selenium, it has now become desirable to minimize the amount of gas phase selenium entering a wet FGD.
Given the above, a need exists for a method that provides for any economical and environmentally suitable method and/or system to control, reduce, mitigate and/or eliminate one or more gas phase selenium compounds and/or one or more other RCRA metals, or RCRA metal compounds (regardless of whether such other RCRA metals or RCRA metal compounds are in the gas phase or some other phase) in one or more of a flue gas stream and/or from one or more pieces of emission control equipment. Additionally, or alternatively, a need exists for a method to control selenium emission and/or one or more other RCRA metal emissions in wastewater streams from one or more pieces of emission control equipment by preemptively controlling, reducing, mitigating and/or eliminating one or more gas phase selenium compounds in one or more of a flue gas stream and/or from one or more pieces of emission control equipment.
In another instance, even in jurisdictions outside of the scope of the US EPA (e.g., other countries and/or multinational jurisdictions such as Europe, China, etc.), a need exists, or will exists, to control the amount of one or more of arsenic (As), barium (Be), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), selenium (Se) and/or silver (Ag) (collectively referred to as the RCRA metals) that is/are emitted in one or more wastewater streams derived from various emission control technologies. Thus, it would be highly desirable to have a technology or method that permits and/or allows for the control of the levels of one or more of arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), selenium (Se) and/or silver (Ag) (collectively referred to as the RCRA metals) that is/are emitted in one or more flue gas streams so as to prevent such one or more RCRA metals from subsequently entering one or more downstream wastewater streams generated during the application of various emission control technologies to various power generation technologies.